1. Field of the Invention
The present invention relates to a magneto-optical recording and/or reading-out apparatus.
2. Description of the Prior
As one of rewritable optical recording mediums, a magneto-optical disc is well known in the art. The magneto-optical disc now available in the market is recorded with information by a method in which, while the magneto-optical disc is rotated in one direction in the presence of a bias magnetic field of a predetermined intensity, a laser beam modulated according to a signal to be recorded is irradiated onto the magneto-optical disc. During the practice of this method, a magnetization of a portion of the magneto-optical disc where a high-power laser beam is irradiated is oriented in a direction conforming to the direction of the bias magnetic field whereas a magnetization of a portion of the magneto-optical disc where a low-power laser beam having a zero or quasi-zero power is irradiated remains changed. In view of this, preparatory to an actual information recording on the magneto-optical disc, the magneto-optical disc must magnetized in a direction counter to the direction of magnetization which takes place during the information recording. This preparatory procedure is known as an erasure which is generally carried out by irradiating a laser beam of a predetermined intensity in the presence of a predetermined magnetic field oriented in a direction counter to the direction of the orientated magnetization which will be established after the information recording on the magneto-optical disc.
As is well known to those skilled in the art, completely flawless magneto-optical discs are not available and most of magneto-optical discs more or less contain defects. Because of this, data recorded on the magneto-optical disc need be read out for verification purpose and then checked to see if an error correction is possible. A process of verification is such that, in the event of an occurrence such an error of a kind that no error correction is a remedy, data to be recorded on that area of the magneto-optical disc where the error has occurred can be recorded on an alternative or replacement area to ensure an accurate information recording or readout.
Accordingly, it is usual for data to be recorded on the magneto-optical disc through the following process.
1) A seeking motion to an address on the magneto-optical disc where they are to be recorded.
2) An erasure of a track region where the recording is to be made.
3) Recording the data on the magneto-optical disc.
4) Verification of the data recorded on the magneto-optical disc.
5) Depending on a result of the verification, an erasure of the alternative or replacement area or re-recording of the data on the magneto-optical disc, followed by a verification of the re-recorded data.
This process sequence is problematic in that a relatively long time is required in making the recording.
In order to eliminate the above discussed problem, an over-write technique in which a data erasure is followed immediately by a data recording is currently known. This over-write technique includes a method such as disclosed in, for example, U.S. Pat. No. 5,020,041 wherein the data recording on the magneto-optical disc is carried out by means of a magnetic field modulated according to a signal to be recorded while being radiated by a laser beam of a predetermined power. Japanese Laid-open Patent Publication (unexamined) No. 62-175948 discloses another method wherein, during the recording, the intensity of the laser beam is modulated according to a signal to be recorded in the presence of a bias magnetic field of a predetermined intensity. Both of these methods requires no erasure process because of the capability of data over-writing and, hence, the following process sequence suffices to accomplish the data recording.
1) A seeking motion to an address on the magneto-optical disc where data are to be recorded.
2) Recording the data on the magneto-optical disc.
3) Verification of the data recorded on the magneto-optical disc.
4) Depending on a result of the verification, an erasure of the alternative or replacement area or re-recording of the data on the magneto-optical disc, followed by a verification of the re-recorded data.
Alternative to the above discussed over-write technique, a technique is also suggested in which the use is made of two optical recording heads, either separate or integrated, so that, while the recording is made by means of one of the optical recording heads, the other of the optical recording heads is used to read out data for verification purpose, such as disclosed in, for example, Japanese Laid-open Patent Publication (unexamined) No. 62-252,552.
According to this publication, the recording completes during each process of making an access to the address where the recording is to be made, erasure of the track on which the recording is to be made and recording of data, or, where the necessity arises, erasing and re-recording onto an alternative area based on a result of verification which is carried out simultaneously with the recording.
However, since with the first mentioned method the erasure takes place simultaneously with the recording and the verification need be performed subsequently, there is a problem in that the length of time that can be reduced is very small.
According to the second mentioned method, the verification takes place simultaneously with the recording and the erasure need be performed before the recording, and therefore, in addition to the problem in that the length of time that can be reduced is very small, there is another problem in that the control of the two lasers requires the optical head to be complicated in structure and expensive to manufacture.
Japanese Laid-open Patent Publication (unexamined) No. 2-139738 discloses a method wherein the first and second mentioned methods are combined. According to this disclosure, while two lasers are mounted on separate or integrated optical heads, a defect is detected simultaneously with the erasure by sensing an abnormality in the light reflected from the magneto-optical disc so that the recording according to a signal to be recorded can be performed by means of the modulated second laser beam without the verification being performed. According to this method, the erasure, the recording and the verification are accomplished substantially simultaneously, and the length of time required to complete the recording can be reduced since the recording can be completed by making an access to the address where data are to be recorded, and the recording of the data or, where the necessity arises, the re-recording of the data in an alternative area based on a result of the verification. However, there is a problem in that the control of the two lasers renders the optical heads to be complicated in structure and difficult or expensive to manufacture. At the same time, since the laser used to record the data and the laser used to detect the defect are installed at different positions, respectively, the position where the data are to be recorded and the position at which the defect detection is made are different from each other. While a correction for this difference may be accomplished naturally by the use of a delay circuit or the like, any possible presence of a variation in rotational speed brings about a problem in that the position where the defect is found in the time-shared data cannot be located.